In cats, the paradoxical phase of sleep occupies about 33 percent of the total sleeping time. Cats which were deprived of paradoxical sleep, by being awakened at its onset, required an increased number of awakenings on successive days of deprivation in order to prevent paradoxical sleep. On the first day of recovery after deprivation, when sleep was not experimentally interrupted, the paradoxical phase occupied 53 percent of the total sleeping time.